A polydimethylsiloxane modified with alkyl groups (except for methyl groups) combines properties of both silicone and alkane and is characterized by excellent characteristics, such as lubricity, humectant (vapor obstructive) properties, adsorption residual properties, etc.
Alkyl-modified dimethylpolysiloxanes, especially those liquid polydimethylsiloxanes that are modified with alkyl groups that contain 4 to 18 carbon atoms (hereinafter referred to as “medium-chain alkyl groups”), are superior in their handling properties to wax-type dimethylsiloxanes modified with long-chain alkyl groups, possess excellent emulsifiability and emulsion stability when used as oiling agents in emulsification systems of cosmetic products, and demonstrate good feeling of application and water repellency. Therefore, it is expected that such alkyl-modified dimethylsiloxanes will find wide application in the manufacture of shampoos, rinsing, hair-treatment, and sun-screen products, moisturization creams, or similar products (see, e.g., Japanese Unexamined Patent Application Publications [Kokai] 2003-12466 and 2003-48813).
Generally, alkyl-modified polydimethylsiloxane is synthesized by causing a hydrosilylation reaction (addition reaction) between polydimethylsiloxane having hydrosilyl groups (Si—H groups) and α-olefin in the presence of a platinum catalyst.
However, an alkyl-modified polydimethylsiloxane synthesized by the above-described method is characterized by a specific (rather unpleasant) odor. One of the sources of this odor is α-olefin, which is used in an excessive amount during synthesis, remains as a residue in the reaction product (crude product), and generates an odor when oxidized.
Since the number of carbon atoms in the α-olefin used for the reaction during synthesis of the liquid polydimethylsiloxane modified with the medium-chain alkyl group is relatively small, i.e., 4 to 18, even though some amount of such α-olefin remains in the reaction product (crude product), it can be removed from the product by heating in vacuum.
However, even if the α-olefin is completely removed (below the limit of detection) from the product (i.e., liquid alkyl-modified polydimethylsiloxane), an unpleasant odor is still sensed and, moreover, this unpleasant odor tends to increase with time.
If the problem associated with such an unpleasant odor is not solved, the liquid polydimethylsiloxane modified with the medium-chain alkyl cannot be used in cosmetics since this odor will negatively affect the quality of scent, and this will significantly limit the scope of practical application of such products.
The present invention is based on the above information.
The first object of the present invention is to provide a method of manufacturing a purified product of a liquid medium-chain alkyl-modified polydimethylsiloxane that is free of a specific (unpleasant) odor and that is practically odorless.
The second object of the present invention is to provide a method of manufacturing a purified product of a liquid medium-chain alkyl-modified polydimethylsiloxane that is odorless and does not exude an unpleasant odor with the lapse of time.
The third object of the present invention is to provide cosmetics that do not produce a specific (unpleasant) odor due to the presence of the liquid alkyl-modified polydimethylsiloxane.